OLED panels produce soft diffuse light with essentially a Lambertian distribution that is inherently pleasing, but efficacy remains a challenge and elevated ambient temperature can drastically reduce lifetime.
The US Department of Energy (DOE) has released another Caliper report, this time focused on OLED luminaires available on the commercial market. The study documents photometric performance, product design considerations, and lifetime reliability through accelerated testing. The four solid-state lighting (SSL) products tested revealed pleasing optical performance, but energy efficiency, lifetime, reliability and cost remain issues.
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The new "Caliper Report 24: Photometric Testing, Laboratory Teardowns, and Accelerated Testing of OLED Luminaires" is the third element in a series of OLED-centric DOE projects to be published this year. Most recently back in July, the agency had issued a report focused on the potential applications for OLED technology in architectural lighting. Earlier in May, the DOE had issued a Gateway report focused on an actual OLED installation in an office.
In Caliper reports, the DOE performs hands-on testing of products available on the commercial market. The DOE buys the products anonymously and installs the luminaires at one of its testing labs such as the Pacific Northwest National Laboratory (PNNL). Generally, the DOE tests some legacy products alongside the SSL products being evaluated. For instance, in a series of Caliper reports on directional PAR38 lamps, the agency has tested six legacy lamps alongside 32 LED-based lamps. But in the OLED report, the agency noted that there were no comparable legacy products in terms of application and form factor. Indeed, the unique thin form remains the reason that the DOE is still funding work and the reason that some companies continue to pursue the technology for general lighting.
The four OLED products tested in the DOE's latest Caliper report showed pleasing optical performance.
The OLED report included testing of a desk lamp from Aerelight, and three products from Acuity Brands — the Chalina (called Revel at launch) ceiling-, pendant- or wall-mountable luminaire, and two versions of the Aedan (called Nomi at launch) sconce. The Aedan product can include one panel emitting light directly, or two panels with the second washing the wall. The Aerelight product uses a panel from OLEDWorks (now the home of the former Philips Lighting OLED unit) and the Acuity products use panels from LG Display.
Generalizing, the DOE said the luminaires tested range in efficacy from 23 lm/W to 45 lm/W and the panels in the products range from 42 lm/W to 55 lm/W. Clearly, the lack of drivers optimized for OLEDs remains an issue as we reported in the article on the Gateway office lighting project. The latest Caliper report documents several different driver concepts. The implementation in the sconces was especially inefficient, with the DOE estimating energy efficiency of 47% to 58%. The Aedan luminaire driver, however, was estimated to be 85% efficiency.
Photometry testing performed with a spectrophotometer goniometer and integrating sphere revealed that all of the products performed closely to manufacturers' specifications. CCT for all was in the 2900K CCT range, although the report says the OLEDWorks panel was specified at 3000K. Moreover, the DOE said the light output produces soft shadows and patterns of light on surfaces with very soft gradients.
There were two other elements of testing in the report. The PNNL performed teardown exercises on the products to examine design practices, for instance in the area of drivers, and RTI International performed accelerated testing to determine reliability and lifetime. In one failed panel, the PNNL suggested that a compromise in the OLED seal along the edge of the glass was the cause of discoloration due to air and moisture interacting with the organic layers.
RTI International tested the products at 45˚C and 75˚C, with rated and high-current input, and with 75˚C temperature and 75% humidity. Even at 45˚C, the panels exhibited faster-than-expected lumen depreciation. Lumen maintenance averaged 87% after 4250 hours of continuous operation at 45˚C. Testing at the higher temperature led to many panel failures, although the failures generally are short circuits — meaning that in a multipanel luminaire, other panels may continue operating when one panels fails.
The Caliper research also revealed other areas in which the OLED sector needs test procedures and technology centric to OLEDs. All of the development for LEDs does not necessarily apply. For example, we already mentioned drivers and in the prior article the fact that LED drivers aren’t a perfect match for OLEDs. For the Caliper report, the DOE tested the products based on the LM-79 standard developed for testing LED luminaires. But the DOE said the standard needs to be modified or extended in places to support accurate and consistent testing of OLEDs. Likewise, there is no equivalent of LM-80 and TM-21 that are used to characterize LEDs and project reliable life. So comparing reliability of data from OLED panel manufacturers is not a precise science the way it has become with packaged LEDs.
You can read the complete Caliper report on OLED technology at the DOE website. The report concludes with describing the OLED sector as still being "in a steep curve of development." The industry needs higher efficacy, longer life, and lower cost as we have reported numerous times. But the DOE said the industry also needs more CCT options, better color maintenance, and more robustness to the environment and even the product shipping process.
